It was put in there to ensure there was an exit mechanism to stop the British complaining. He never imagined that someone would actually use it.

"My intention was that it should be a classic safety valve that was there, but never used. It is like having a fire extinguisher that should never have to be used. Instead, the fire happened.

He says that the other EU members will make sure that they make the Brexit talks as tough as possible to show Britain the error of it's ways and too make sure that anyone else thinking of leaving understand how hard and damaging it will be for them. In a sense for the EU to survive they have to.